I. Field of the Invention
The present invention relates generally to direct injection internal combustion engines and, more particularly, to a method and apparatus for reducing engine noise, especially at low engine speeds.
II. Description of Related Art
Direct injection internal combustion engines of the type used in automotive vehicles have enjoyed increased popularity due in large part to their fuel economy. In a direct injection engine, the fuel injector is mounted in the engine block and has its fuel injection outlet end open directly to the internal combustion chamber. Consequently, upon activation or opening of the fuel injector, the fuel is injected directly into the internal combustion engine, rather than upstream from the fuel intake valves as in the previously known multi-point fuel injectors.
In order to supply fuel at a sufficiently high pressure to overcome the high pressures of the combustion chambers, these previously known direct injection engines include a high pressure fuel pump having an inlet connected to a fuel source such as the fuel tank, and an outlet open to a fuel rail. The fuel rail, in turn, is fluidly connected to the engine fuel injectors.
The previously known high pressure fuel pumps used with direct injection engines typically include a plunger that is reciprocally driven by a multi-lobe cam. An inlet valve is fluidly disposed in series between the fuel pump inlet and the fuel source while an outlet valve is fluidly connected in series between the fuel pump and the fuel rail. During reciprocal movement of the plunger, the plunger inducts fuel through the fuel inlet valve when the plunger moves in a first direction, and conversely the fuel pump pumps fuel out through the outlet valve to the fuel rail upon movement of the plunger in the opposite direction.
One disadvantage with the previously known direct injection engines, however, is that such engines tend to be noisy, especially at low engine speeds such as less than 1,000 rpm. The engine noise, furthermore, is largely attributable to three separate events.
More specifically, the fuel injectors themselves create noise when activated or opened due to the high pressure fuel injection. This high pressure fuel injection is oftentimes accompanied by noise causing vibration of various engine components.
The opening of the fuel inlet valve in the high pressure fuel pump also creates noise. Similarly, the opening of the outlet valve from the high pressure fuel pump also creates engine noise.
In the previously known direct injection engines, the opening of the fuel inlet valve to the fuel pump, the opening of the fuel outlet valve in the fuel pump, and the opening of the fuel injectors all occur at different crank angles of the engine crankshaft. For example, as shown in FIG. 1, graph 10 illustrates the noise created by the opening of the fuel injectors for a six cylinder direct injection engine. Graph 12 illustrates the noise output from the outlet valve of the high pressure fuel pump while graph 14 illustrates the noise generation by the inlet valve of the high pressure fuel pump.
Graphs 10-14 are illustrated as a function of the crank angle 16 of the engine crankshaft and the cam angle 18 of a multi-lobe or triangular cam used to drive the fuel pump plunger. Graph 19 illustrates the angle or position of the fuel pump plunger.
Graph 20 illustrates the total noise produced by the direct injection engine. As can be seen from graph 20, the total noise includes a separate noise peak corresponding to the fuel injector opening, the pump outlet valve opening, and the pump inlet valve opening. This noise, furthermore, is particularly noticeable to occupants of an automotive vehicle at low engine speeds, such as less than 1,000 rpm.